Petards

"Hoist with his own petard," an ancient phrase
signifying that one's carefully laid scheme has exploded, had truly
graphic meaning in the old days when everybody knew what a petard was.
Since the petard fired no projectile, it was hardly a gun. Roughly
speaking, it was nothing but an iron bucket full of gunpowder. The
petardier would hang it on a gate, something like hanging your hat on a
nail, and blast the gate open by firing the charge.

Small petards weighed about 50 pounds; the large
ones, around 70 pounds. They had to be heavy enough to be effective, yet
light enough for a couple of men to lift up handily and hang on the
target. The bucket part was packed full of the powder mixture, then a
2-1/2-inch-thick board was bolted to the rim in order to keep the powder
in and the air out. An iron tube fuze was screwed into a small hole in
the back or side of the weapon. When all was ready, the petardiers
seized the two handles of the petard and carried it to the troublesome
door. Here they set a screw, hung the explosive instrument upon it, lit
the fuze, and "retired."

Petards were used frequently in King William's War of
the 1680's to force the gates of small German towns. But on a
well-barred, double gate the small petard was useless, and the great
petard would break only the fore part of such a gate. Furthermore, as
one would guess, hanging a petard was a hazardous occupation; it went
out of style in the early 1700's.